docs: backport runnable code example fixes to v3 versioned docs

[vdusek]

Backports the documentation fixes from #1002 into the v3 versioned docs (website/versioned_docs/version-3.4/). The original PR only updated the next (docs/) docs, so the released v3 guides were left out of sync.

The changes were applied by extracting #1002's diff and re-pointing it at the versioned tree. Every touched file is now byte-identical to its docs/ counterpart.

What it carries over:

• Lower the page cap from 50 to 10 across all crawling examples, so the browser-based ones finish within the 180s runnable-demo timeout.
• Selenium: slim the runnable example to a plain crawler, and move the proxy-auth extension into a separate, non-runnable snippet (code/04_selenium_proxy.py).
• Browser Use: make it non-runnable (needs an LLM API key the shared runner can't provide); same note added to the Scrapy and Scrapling-browser examples.
• Pydantic: log a readable validation summary and fail cleanly via Actor.fail.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (482e725) to head (15d446c).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1004   +/-   ##
=======================================
  Coverage   91.61%   91.61%           
=======================================
  Files          49       49           
  Lines        3171     3171           
=======================================
  Hits         2905     2905           
  Misses        266      266           

Flag	Coverage Δ
e2e	35.69% <ø> (ø)
integration	56.41% <ø> (ø)
unit	80.51% <ø> (ø)

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